Machines for milling

​Previously, machines could be split into four categories – horizontal or vertical, and turning or milling.

Today, machines are developing in all directions. Turning centres now have milling capability due to driven tools, and machining centres have turning capability – turn mill or mill turn machines. CAM developments mean that
five-axis machines are increasingly common. The results of these trends
create new demands and opportunities for tooling:

• Increased flexibility
• Fewer machines/set-ups to complete a component
• Reduced stability
• Longer tool lengths
• Lower depth of cuts.

 

 

 

Horizontal:

• Favourable for milling larger components.
• Facilitates chip evacuation in cavity milling, and prevents re-cutting.
• Less mass to accelerate/decelerate.
• Often, four axes provide access to three sides.
• Ergonomic and economic pallet technology.
• Most common machine type for use of side and face milling cutters.
  

Small vertical machining centers:

• Small total envelope, requires little space in the workshop.
• Well suited to high speed/feed – light and fast.
  

Large vertical machining centers:

• Provide better stability while the workpiece is resting on the table.
• Suitable for larger and heavier workpieces.
• Column types for huge components.
• Can work with longer and heavier tool set-ups.​

 

 

 

​The condition and stability of the machine have an effect on the quality of the surface, and can also impair tool life. Excessive wear on the spindle bearings or feed mechanism can result in a poor surface structure.

The stability of the entire tool set-up is of outmost importance. Factors such as tool overhang, Coromant Capto coupling, tuned adaptors, etc. should be considered.

 

 

 

​Basically, the power requirements in milling vary along with the:

• amount of metal to be removed
• average chip thickness
• cutter geometry
• cutting speed.
  

The greater the metal removal rate (Q cm³/min), the higher the power requirement.
Low spindle speeds for roughing of exotic materials place great importance on the availability of sufficient power and torque.

A machine with insufficient torque and power will produce fluctuating chip thickness, which in turn causes unstable performance.

The majority of modern machining centres have direct driven spindles. Ever increasing spindle speed capacity and/or capability results in:

• Lower torque at higher rpm
• Lower power at lower rpm
  

Therefore, machines with high rpm capabilities have limitations for roughing wíth larger diameter cutters, which require low rpm and high power.

Machining strategies need to be adapted. This explains the trend in light and fast machining – which uses a smaller cutter diameter, small depth of cut, a_p/a_e, and high feed per tooth, f_z.

Machines for components requiring high power at low rpm can be geared to produce an optimum performance for both roughing and finishing.

 

 

 

​ISO 30, 40, 50 and 60 spindles have natural built-in advantages and limitations.

Heavy roughing requires a larger spindle, whereas high speed milling requires lower torque, making a smaller spindle more suitable.

The size of the spindle will define the maximum milling cutter diameter and the depth of cut that the machine is capable of handling.

Although there are exceptions, due to varying machine tool conditions, a general rule for selecting the cutter size is:

ISO 60 – “larger cutters”.
ISO 50/Coromat Capto size C8 – D_c 160 mm.
ISO 40/Coromat Capto size C6 – D_c 100 mm.
ISO 30/Coromat Capto size C4 – D_c 50 mm.

Components requiring long edge cutters require, at a minimum, an ISO 50 or Coromant Capto size C8 spindle.

Tool coupling integrated in the spindle provides the best stability.

On gantry machines and other larger machine tools, cutters may be direct mounted on the spindle nose, which provides extreme stability and the smallest possible protrusion.